The present invention concerns a regeneration process for a catalyst containing at least one metallic element which is platinum, palladium, ruthenium, rhodium, osmium, iridium or nickel, preferably platinum, on a refractory oxide based support, which has been deactivated by coke deposition. The regeneration process is such that it produces an essentially uncoked catalyst using a controlled coke combustion process which at least restores the performance of the catalyst, ie., the activity, selectivity and stability of the regenerated catalyst are at least equal to that of the initial uncoked catalyst. The present invention is of particular application to regenerating dehydrogenation and dehydrocyclization catalysts for hydrocarbon feedstocks comprising mainly paraffins containing 3 to 8, in particular 3 to 5 carbon atoms per molecule.
Dehydrogenation catalysts are conventionally composed of platinum, optionally tin, optionally an alkali metal, and optionally a halogenated compound on a refractory oxide such as alumina. Platinum and tin based supported catalysts for dehydrogenating paraffin hydrocarbons are described in U.S. Pat. No. 3,531,543 U.S. Pat. No. 3,998,900.
Dehydrogenation reactions are endothermic and reversible. The conversion ratios are limited by the thermodynamic equilibrium conditions. Severe conditions favorably displace the dehydrogenation reaction towards olefin formation, but also favor undesirable secondary coke-forming and/or cracking reactions. The presence of coke is the main source of dehydrogenation catalyst deactivation. In certain instances, sintering and/or poisoning of the metallic phase can also result in loss of catalyst performance. A regeneration process is thus necessary in order to restore the initial catalytic properties of the catalyst.
In the case of dehydrogenation processes, the deactivated catalysts are regenerated using a process which includes a combustion step for the coke present on the catalyst by treating the latter with oxygen at a high temperature between 400.degree. C. and 600.degree. C. to burn off the hydrocarbon species constituting the coke. However, agglomeration of metallic particles during this step reduces the active surface area of the metal and thus reduces the activity and stability of the regenerated catalyst. As a consequence, a metallic phase redispersion step is required following the combustion step. For this type of catalyst, this is conventionally an oxychlorination step which is carried out by treating the catalyst with a chlorine-containing gas in the presence of oxygen and, if necessary, water, using any technique known to the skilled person.
U.S. Pat. No. 5,087,792 describes a three step regeneration process for a coked dehydrogenation catalyst. The first step involves burning off the coke by treating the catalyst with a gas containing oxygen at a temperature of between 471.degree. C. and 538.degree. C. The second step consists in drying the catalyst, for example in dry air between 426.degree. C. and 593.degree. C. Finally, the third step involves redispersing the platinum on the catalyst surface by treatment with a chlorine-containing gas [0.01 to 0.2% (molar) of chlorine in air].
U.S. Pat. No. 4,359,400 describes the regeneration of a coked platinum based catalytic reforming catalyst which is carried out in a plurality of steps. The first step involves burning off the coke by treating the catalyst with a gas containing oxygen at a temperature close to 482.degree. C. The second step consists in reducing the catalyst by treatment in hydrogen at about 482.degree. C. The third step involves treating the catalyst with a gas containing a hydrogen halide at the same temperature. The fourth step involves treating the catalyst with a gas containing elemental halogen at about 482.degree. C. Finally, the catalyst is reduced in hydrogen at 482.degree. C. in the fifth step.
U.S. Pat. No. 3,875,049 describes the regeneration of a deactivated platinum and tin based catalytic reforming catalyst using a two step process. The first, coke combustion, step is carried out by treatment with a gas containing oxygen at 750.degree. F. In a second step, the catalyst is treated with a gas containing oxygen, carbon tetrachloride and water to reactivate the catalyst.
These examples show that regeneration of catalysts containing at least one metallic element selected from platinum, palladium, ruthenium, rhodium, osmium, iridium or nickel, preferably platinum, on a refractory oxide based support, is well known but requires a large number of steps. These steps are most often carried out in at least two different reactors or in at least two different zones in the same reactor, with concomitant disadvantages regarding the industrial process.